The present disclosure relates to methods and apparatus for converting mechanical power into electrical power. More particularly, the invention relates to devices and methods that convert mechanical power of an intermittent pressure exerted by a flexible pipe or vessel, such as a blood vessel within a living body, into electrical power.
The operation of various subcutaneously implantable medical devices such as cardioverter-defibrillators, cardiac pacemakers, neurostimulators, medical monitoring devices and drug infusion devices requires electrical power typically provided by long life stored energy systems such as Lithium Iodine (Li/I2), Lithium Silver Vanadium Oxide (Li/SVO), and Lithium Carbon Monofluoride (Li/CFx) batteries. The voltages provided are substantially constant over a period ranging typically from a few months to a few years and are available using current electronic technologies. For example a typical conventional Lithium Iodine battery used in a cardiac pacemaker device may provide a voltage between 2.5 and 3.3 volts and a current of a few microamperes for a period of 6 to 10 years.
Batteries for different implantable applications may have different characteristics. For example, a lithium silver vanadium oxide battery may be able to support the high current bursts of a few amperes required by an implantable cardioverter defibrillator. The Lithium Iodine and Lithium Carbon Monofluoride batteries are more suited for use in cardiac pacemakers, neurostimulators, and drug infusion devices where the electrical load ranges from microamperes to milliamperes.
Conventional power systems for implantable medical devices are not permanent in the sense that once the amount of stored energy diminishes below a certain acceptable level they need to be replaced with a new power system. The procedure to replace the power system typically requires surgery.
The size of the power system is a major determinant of the size of the implantable medical device it powers in the sense that the size of the power system may comprise one third to one half of the size of the implantable medical device. Since it is desirable for the well being of the patient to minimize the number of required surgical procedures, the size of the implanted power system must be sufficient to provide energy to the implantable medical device for as long of a period as possible.
The period between surgeries is then related to the storage ability of the technology used by the power system and the power requirements of the implantable medical device. The size of the implantable medical device is then related to the level of the power system and electronics technologies available at the time of the device implantation. For example, today the smallest pacemaker devices may be 1.5″ in diameter and 0.3″ thick. Other implantable medical devices may be considerably larger.
Furthermore, a new class of implantable medical devices comprises in-vivo medical monitoring devices requiring reliable long term power systems. Examples of long term in-vivo medical monitoring devices include, for example, implantable glucose monitoring devices and implantable blood pressure monitoring devices.
Sometimes it is common for a patient to require more that one implantable medical device. For example, a paraplegic patient may require multiple neurostimulant devices and a pacemaker or defibrillator device or an implantable insulin pump. The use of multiple implantable medical devices is expected to increase with the development of new smaller size medical devices based on micro-electro-mechanical-systems (MEMS) and bionanotechnology systems.
Therefore, in many applications it may be desirable to power an implantable medical device with a power system that is permanent in the sense that in normal operation it does not require maintenance or replacement. Such a permanent power system may not require surgery or other procedure that brings risk to the patient.
It may also be desirable for a power system for implantable medical devices to be of very small size, for example, small enough to be part of an implantable medical device that can be attached to a significant artery or vein. By means of example, an implantable medical monitoring device may use such a power system to monitor the blood pressure in an artery or vein.